Large-scale changeable message sign structures are known in the art. Typically, such sign structures present a front face assembly the outer layer of which is an opaque panel with an array of regularly spaced openings therein. This layer, known as an aperture mask, has aligned behind it an array of electronically controlled sign pixel elements, visible through the aperture mask openings. In current practice, the pixel elements are often clusters of light-emitting diodes (LEDs). The operation of the pixel elements is electronically controlled by a controller and appropriate circuitry to present a desired message to viewers, such as passing motorists. Typically, interposed between the aperture mask and the sign pixel elements is a window panel, the purpose of which is to prevent the ingress through the mask openings of potentially damaging environmental agents, such as water, dust, salt spray, insects and the like, into the sign interior.
From time to time, it may be necessary to service or replace the aperture mask panel and/or window panel, as they can deteriorate with age (for example, ultraviolet exposure may damage a window panel) or be damaged by various occurrences, such as accidents, severe weather conditions or even acts of vandalism. It is thus desirable to be able to have access to these panels for the purpose of inspection, servicing or replacement.
In some prior art structures, the aperture mask and/or window panel are attached to the sign structure from the exterior and thus can only be accessed from the sign exterior. In other prior art structures, although it may be theoretically possible to access the aperture mask and/or window panel from the interior, it may be difficult or impractical, particularly if a significant amount of internal sign structure must first be removed. For example, as shown in FIG. 12 of U.S. Pat. No. 5,497,573, some prior art aperture mask panels can be relatively large. This fact suggests that significant internal sign structure must be removed to access the panels. Moreover, their sheer size may make it preferable, for practicality or safety reasons, to handle them from the exterior. In addition, seams between panels are required to be caulked or sealed, a step which may be better accomplished from the exterior. For any such structures, to access the sign exterior, it may be necessary to close the highway, either partially or completely, thus interfering with traffic flow and potentially increasing risk for motorists and at least causing inconvenience.
In other prior art structures, such as shown in U.S. patent publication no. U.S. 2004/0123501 and U.S. Pat. No. 6,741,222, aperture mask modules or related components are bolted together in a manner that makes it impossible or at least impractical to remove and replace individual aperture mask modules and window panels. For example, in the structure of U.S. Pat. No. 6,741,222, to remove a particular mask panel, many, if not all, of the components shown in FIGS. 4 and 5 would have to be removed not only from that panel but also from the neighboring panels on each side, so as to access the bolts (shown in FIG. 10 thereof) holding such panels together. In addition to the time and effort needed to disassemble and then later re-assemble such a structure, there is increased risk of damage to components or error in re-assembly. In some versions of these structures, a vertical support bar and/or sealant material is provided between adjacent channel-shaped aperture mask modules, such that, even if it were possible to remove a single mask module from the interior of the sign, there would be a danger that in the removal process the vertical bar and/or gasket material might fall to the road surface below, thus posing a hazard; moreover, in replacing such a removed module, it would be extremely difficult or impossible to replace the vertical bar and gasket material. The silicone sealant material specified in U.S. Pat. No. 6,741,222 can only be applied from the sign exterior, a fact which essentially eliminates any advantage in attempting to remove and replace a mask module from the sign interior.
As noted above, some prior art structures require the use of sealant materials, such as silicone, between aperture mask panels or modules. Some common sealant materials, particularly silicone, are known to degrade with time, especially when exposed to sunlight. Such signs thus suffer from the additional disadvantage that they will require additional maintenance (likely from the sign's exterior) from time to time to replace or repair such sealant material.
Moreover, some prior art structures suggest that the same sealant material may be used with other components of the structure, for example to seal an aperture mask panel to a window panel. The problem in doing so is that the sealant, again particularly silicone sealant, can be difficult to remove, thus making it more difficult to effectively service a damaged panel structure.
There thus remains a need for a sign structure which allows convenient access from a sign's interior to the aperture mask and window panels so as to allow the servicing thereof.